Method of encapsulating transformer



2 Sheets-Sheet 1 IN VEN TOR.

H. REBER METHOD OF ENCAPSULATING TRANSFORMER l l I 1 l h 1 l l I l l -jSept. 20, 1966 Filed July 29, 1963 (Z5 BY Haber?? Heber @www Sept. 20,1966 H. REBER 3,274,320

METHOD OF ENCAPSULATING TRANSFORMER IN VEN TOR.

Q) BY 7g2/56M @aber awww.)

United States Patent C) 3,274,320 METHOD F ENCAPSULATING TRANSFRMERHubert Relier, Zanesville, Ohio, assigner to McGraw- Edison Company,Milwaukee, Wis., a corporation of Delaware Filed .Iuly 29, 1963, Ser.No. 298,103 3 Claims. (Cl. 264-135) This invention relates toencapsulated electrical transformers and more particularly to a methodof constructing encapsulated coils for electrical distributiontransformers.

It is known in the manufacture of electrical transformers to encapsulatethe coils in an insulating resin jacket so they will be substantiallyunaffected by moisture, atmospheric conditions, and shock. Onedisadvantage of conventional encapsulating methods is that airfrequently becomes entrapped in the interstices of the coil. Such airinclusions, or voids may cause corona and result in impairment of theelectrical characteristics of the transiformer. Such air inclusions canbe reduced by impregnating the coil with a liquid insulating dielectricbefore the encapsulating step. However, cracks are often caused in theencapsulating resin jacket during temperature cycling of the transformerdue to differences in the coellcients of expansion of the metal parts ofthe coil, the encapsulating resin, and the impregnating dielectric usedto displace the air. The coeiicient of expansion of the impregnatingdielectric is higher than that of -any of the other components of thecoil, and consequently the mechanical stresses tending to crack theresin encapsulating jacket increase in proportion to the amount ofinsulating dielectric in the coil.

It is an object of the invention to provide a method of constructingencapsulated coils for electrical transformers which positions thehighest dielectric strength insulation at the places within the coilwhere the voltage stress is greatest and fills the interstices withinother portions of the coil with encapsulating resin, whereby themechanical stresses due to differences in rates of expansion between theimpregnating and encapsulating materials tending to crack theencapsulating jacket are minimized.

It is a further object of the invention to provide a method ofconstructing encapsulated coils for electrical transformers whichdisposes porous insulating material impregnated with liquid dielectricin the barrier, interlayer, and exterior insulation of the coil andencapsulating resin within other portions of the coil so that mechanicalforces tending to crack the encapsulating jacket are minimized.

These and other objects and advantages of the invention will be morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawing wherein:

FIG. l is a front elevation View of an encapsulated transformerconstructed in accordance with the method of the invention;

FIG. 2 is a side elevation view of the transformer shown in FIG. l;

FIG. 3 is a view taken on line 3-3 of FIG. l; and

FIG. 4 is a vertical section view through an enclosure suitable for thesteps of inpregnating and encapsulating the coils of the transformer ofFIG. 1.

The method of the invention is particularly adapted for the constructionof encapsulated coils for electrical distribution transformers. Theentire transformer core and coil assembly may be encapsulated as a unitin a thermosetting resin jacket or, as illustrated in the drawing, thecoils may be constructed separately and encapsulated with a resin jacketbefore they are assembled with the laminations of the magnetic core. AU-shaped metallic mounting bracket adapted to be mounted on a pole (notshown) 3,274,320 Patented Sept. 20, 1966 ice supports a transformer coreand coil assembly 11. The transformer core and coil assembly 11 includesa pair of cylindrical coils 12 of rectangular cross section disposed inside-by-side relation and embedded in a thermosetting resin jacket 13. Avertically elongated window may be provided in jacket 13 between thecoils 12 to facilitate cooling. The coils 12 have vertically extendingaxial openings 14 through which the laminations of magnetic core 15 areassembled after the coils l2 are embedded in resin jacket 13. Magneticcore 15 preferably comprises upper and lower complementary U-shapedhalves of nested U-shaped laminations 16U and 16L which are insertedinto the axial openings 14 in the coils 12 and abut in core joints (notshown) within the axial openings 14. Upper and lower core clamps 17U and17L are secured to and urged against the U-shaped core halves 16U and16L lby banding straps 18 which surround the mating U-shaped core halves16U and 16L and hold the Icore laminations in abutting relation.lInternally threaded studs 19 welded to the upper and lower core clamps17U and 17L are aixed to the mounting bracket r1.0 by bolt means 22.Stub cables 20 molded within resin jacket 13 adjacent the upper end ofcoils 12 during the encapsulating step are electrically connected to theprimary winding sections of coils 12. A plurality of secondary terminals21 affixed within inserts 25 molded in resin jacket 13 adjacent thelower end of coils 12 are electrically connected to the secondarywinding sections of coils 12.

Coil 12 may be wound on a suitable mandrel and includes tubular innerinsulation 23 cover-ing the inner periphery thereof which electricallyisolates the windings from the magnetic core 15. Inner insulation 23 mayinclude a plurality of layers of a film of insulating material (notshown) which facilitates release of the coil from the mold afterencapsulation, 1a plurality of layers of fiberglass (not shown) in matform to provide a moisture seal after encapsulation, and a plurality oflayers of suitable cellulosic insulating material such as Kraft, rag, ormanila paper which has preferably been impregnated with phenylenediamine to increase the thermal stability as disclosed in the copendingapplication of Fred S. Sadler, Serial No. 268,401 having the sameassignee as the present invention. As described hereinafter suchthermally stabilized cellulosic insulating material is preferably usedin the barrier, interlayer, and inner `and outer wrappings for the coil12. A wrapping of outer insulation 24 covering the exterior of coil 12may comprise a plurality of layers of such thermally stabilizedcellulosic insulating material (not shown) and a layer of fiberglasstape. Each coil 12 may comprise a radially outer primary winding section26 separated by a high-low insulating barrier 27 from a secondarywinding section 28 radially inward therefrom. Insulating barrier 27preferably comprises a plurality of layers of such thermally stabilizedporous cellulosic insulating material wound Ito a sufficient thicknessto provide the desired dielectric breakdown strength between primarywinding 26 .and secondary winding 28. Primary winding 26 may include aplurality of layers of conductor turns 30, 31 and 32 separated byinterlayer insulation 33 of such thermally stabilized, porous cellulosicinsulating material disposed between the barrier 27 and the outerinsulation 24. Secondary winding 28 may include =a plurality of layersof conductor turns 35, 36 and 37 separated by layers 33 of thermallystabilized, porous cellulosic insulating material between the innerinsulation 23 and the barrier 27.

In Iaccordance with the invention, the portions of coil 12 subjected tothe highest voltage stress, i.e., the barrier, interlayer, and the inner`and outer insulations are constructed of porous insulating material,preferably thermally stabilized cellulosic insulation, impregnated witha suitable insulating ydielectric such as impregnating oil. The coils 12are dried in an oven lat approximately 250 F. for sixteen hours toremove moisture. The coils may -then be removed from the oven andpositioned within a hollow container 40 of a suitable impregnating unit.The interior of container 40 is then `sealed off from the atmosphere bymeans includin-g Ia cover 41, the interior of container 40 is connectedthrough a valve 42 to a conduit 43 communicating with a vacuum pump 48driven by a mot-or 45, and a vacuum .of a few millimeters of mercury isdrawn on container 40 to evacuate through conduit 43 the moisture from-the coils 12 for approximately four hours. A suitable insulatingdielectric 44 such as transformer oil is preheated -to approximately 180F. and subjected to a vacuum of a few millimeters of mercury forapproximately four hours. While a vacuum is still being drawn oncontainer 40, the insulating dielectric liquid 44 may be admitted intoevacuated container 40 through a conduit 46 connected thereto through avalve 47. The impregnating oil 44 is drawn into container 40 to a levelcompletely covering the coils 12. The impregnating oil 44 flows into thecoils 12 and displaces the air from and lls all of the interstices ofcoils 12 wherein air had been entrapped. Preferably the impregnatingcycle is continued for an hour while vacuum is continually drawn on thecontainer 40. Air at a pressure of approximately ninety pounds persquare inch is then introduced into chamber 40 above the impregnatingliquid 44 to force the oil into the innermost interstices within thecoils 12.

After the impregnating step is completed, the coils 12 lare removed fromthe bath of impregnating oil 44 within container 40, and theimpregnating toil 44 is permitted to drain off from coils 12. A-t firstthe draining of impregnating oil 44 is in a steady stream as the oilflows from within the spaces between adjacent copper turns in theconductor layers 30-32 and 35-37 and from the voids and pockets withinthe coils 12 such las the spaces between the overlapping layers of theend -turn reinforcements (not shown). The draining step is continueduntil all impregnating oil 44 has owed from the coils 12 except thatretained by capillary attraction within the pores of the porousinsulation members such as 23, 24, 27 and 33 and the oil 44 falls slowlyin drops from the coils 12. The optimum dripping time is dependent uponmany variables such as the kva. size of the transformer, the operatingvoltage, the method of winding, the size of the pores in the insulatingmaterial, the Iviscosity of the impregnating oil, and the viscosity andtemperature of the encapsulating resin. In general, the dripping time isproportional to kva. size of the transformer and to the primary voltageof the transformer, and `for a 7200 volt transformer with thermallystabilized paper insulation and transformer oil as the impregnatingdielectric and an encapsulating resin of 100 centipoise viscosity, thefollowing dripping times were found to be satisfactory -at 20 C. ambienttemperature:

Dripping time,

Transformer kva.: minutes Less than 1 5 The optimum dripping time isinversely proportional to the viscosity of the encapsulating resin usedto form the resin jacket 13, and the following dripping times were foundsatisfactory for a l kva., 7200 volt transformer with thermallystabilized paper insulation and transformer oil as the liquid impregnantat 20 C. ambient temperature.

Viscosity of encapsulating resin: Dripping time.

Above 1000 centipoises Less than 5 minutes. 1000 to 50 centipoises 5 to60 minutes. Below 20 centipoises Apply heat.

The optimum dripping time is proportional to the viscosity of theimpregnating oil and inversely proportional to 4the viscosity of theencapusulating resin. The dripping time may be shortened by heating theimpregated coil 12 during the draining step to thereby lower theviscosity of the impregnating oil. For example, lthe coil 12 may beheated during the draining step to approximately the temperature atwhich the encapsulating resin will be cured, and with encapsulatingresins of low Viscosity it was found desirable to heat the core and coilassembly to approximately 250 F. during the draining step. The time ofdraining is not critical and should be continued until all impregnatingliquid 44 is removed from the coils 12 except that retained by capillaryattraction within the pores of the paper insulation 23, 24, 27, and 29and the oil falls in slow drops from the coils 12.

The coils 12 are then placed side-by-side in a mold 49 (shown in outlineonly in dotted lines), crossover connections may be made between thecoils 12, and conductors from the primary and secondary winding may besuitably positioned in the mold. The mold containing the side-by-sidecoils may then be placed in a suitable chamber such as container 40, thecover 41 placed thereon, the container sealed, and a vacuum of a fewmillimeters of mercury drawn on the container. A liquid thermosettingencapsulating resin such as a low viscosity epoxy or ethoxylene resincontaining an appropriate hardener such as methyl nadic anhydride and asuitable filler such as slate iour may then be introduced into the moldwhile vacuum is being drawn on the container. The encapsulating resinfills all the spaces within the coils 12 in which the impregnating oil44 is not retained by capillary attraction and thoroughly envelops thecoils 12. The liquid encapsulating resin displaces the air from allin-terstices, crevices, and openings in the coils 12 except the poresWithin the paper insulation which are small enough 4to retain theimpregnating oil 44 by capillary attraction. A vacuum may be drawn onthe container for approximately `two hours after the introduction of theencapsulating resin, and then air at a pressure of ninety to one hundredpounds per square inch pressure may be introduced into container 40. Thecoils 12 are then heated for approximately sixteen hours at F., therebycausing the encapsulating resin to solidify into enclosing jacket 13.The coils 12 so enclosed in the solidiiied resin jacket 13 are thenremoved from the mold and baked in an oven at an elevated temperature toconvert the encapsulating resin into an infusible solid. For example,the encapsulated coils 12 may be baked at approximately 250 F. fortwenty-four hours to cure the epoxy resin and convert it into theinfusible state. The laminations of magnetic core 15 are then insertedinto the axial openings 14 in coils 12 and secured therein by bandingstraps 18.

It will be appreciated that the places within the coils 12 such as thebarrier 27, layers 33 between conductor turns, and inner and outercoverings 23 and 24 which are subject to the highest potential stressare insulated by material of maximum dielectric strength, namely, porousinsulation, impregnated by transformer oil. Crevices cracks, spaces, andopenings within the Coils 12 larger than the pores which retain theimpregnating oil by capillary attraction are filled by the encapsulatingresin. Only the optimum amount of impregnating oil desirable forpurposes of insulation is retained within the coils 12, and consequentlythe mechanical stresses tending to crack'the resin jacket 12, arisingfrom the differences in the coefficients of expansion of encapsulatingand impregnating materials, are minimized. Further, since coils 12 arenot in the mold during impregnation as is required with other knownencapsulating processes, the impregnating oil does not interfere withrelease of the solidied resin jacket from the mold.

While only a single mode of practicing the invention has beenillustrated and described, many modifications and variations thereofwill be readily apparent to those skilled in the art, and consequentlyit is intended in the appended claims to cover all such modications andvariations which fall within the true spirit and scope of the invention.

I claim:

y1. In the method of constructing an electrical trans- Iformer includinga magnetic core linked by an encapsulated cylindrical coil havingconcentric llayers of conductor turns, the steps of providing porousinsulation between said conductor layers of said coil, immersing saidcoil in a liquid insulating dielectric Iimpregnant, whereby said-impregnant displaces air Within the interstices of said coil, drainingsaid liquid impregnant from said coil except that retained by capillaryattraction within the interstices in said porous insulation, envelopingsaid coil in thermosetting encapsulating resin in liquid form, andcuring said thermosetting resin.

l2. In the method of constructing an electrical transformer including amagnetic core linked by an encapsulated cylindrical coil havingconcentric layers of conductor turns, the steps -of providing porousinsulation between said conductor layers of said coil, immersing saidcoil in a liquid insulating dielectric impregnant, draining said liquiddielectric impregnant from said coil except that retained by capillaryattraction in said porous insulation and for a period of time until saidliquid impregnant falls slowly in drops from said coil, enveloping saidcoil in a thermosetting encapsulating resin in `liquid form, and curingsaid thermosetting encapsulating resin, said period of time for drainingsaid impregnant from said coil being a -function of the kva. size ofsaid transformer and being inversely proportional to the viscosity ofsaid encapsulating resin.

3. In the method of constructing .an electrical transformer including amagnetic core linked by an encapsulated cylindrical electrical coilhaving :primary and secondary windings each comprised of concentriclayers of conductor turns, the steps of providing .porous insulationbetween said conductor layers of each said winding and between saidprimary and secondary windings and at the internal and exterior surfacesof said cylindrical coil, immersing said coil in a liquid insulatingdielectric impregnant, draining all of said liquid insulating dielectricimpregnant from said coil except that retained by capillary attractionWit-hin said porous insulation and for a time until said liquidimpregnant falls slowly in drops from said coil, placing saidtransformer in la mold, introducing a thermosetting encapsulating resinin liquid [form int-o said mold until it envelops said coil, and curingsaid encapsulating resin.

References Cited by the Examiner UNITED STATES PATENTS 2,459,018 l/ 1949De Monte et al. 264-272 X 2,656,290 10/ 195 3 Berberich et all. 264-27-2X 2,857,626 10/195'8 Wagner et al 264-272 X 3,071,496 1/1963 Fromm et al264-272 `X 3,233,311 2/ 1966 Giegerich et al. 264-272 X ROBERT WHITE,Primary Examiner.

L. S. SQUIRES, Assistant Examiner.

1. IN THE METHOD OF CONSTRUCTING AN ELECTRICAL TRANSFORMER INCLUDING A MAGNETIC CORE LINKED BY AN ENCAPSULATED CYLINDRICAL COIL HAVING CONCENTRIC LAYERS OF CONDUCTOR TURNS, THE STEPS OF PROVIDING POROUS INSULATION BETWEEN SAID CONDUCTOR LAYERS OF SAID COIL, IMMERSING SAID COIL IN A LIQUID INSULATING DIELECTRIC IMPREGNANT, WHEREBY SAID IMPREGNANT DISPLACES AIR WITHIN THE INTERSTICES OF SAID COIL, DRAINING SAID LIQUID IMPREGNANT FROM SAID COIL EXCEPT THAT RETAINED BY CAPILLARY ATTRACTION WITHIN THE INTERSTICES IN SAID POROUS INSULATION, ENVELOPING SAID COIL IN THERMOSETTING ENCAPSULATING RESIN IN LIQUID FORM, AND CURING SAID THERMOSETTING RESIN. 